This invention relates to a process for oxidizing H2S contained at low concentration in a gas directly to sulphur by catalytic means in the vapour phase.
In order to recover H2S contained at low concentration, namely a concentration of less than 25% by volume and, more particularly, between 0.001% and 20% and most especially ranging from 0.005% to 10% by volume in gases from various sources, use may especially be made of processes involving a direct catalytic oxidation of H2S to sulphur according to the reaction:
H2S+1/2O2xe2x86x92S+H2O.
In such processes, the H2S-containing gas to be treated, mixed with a suitable amount of gas containing free oxygen, for example, air, oxygen or oxygen-enriched air, is brought into contact with a catalyst for oxidizing H2S to sulphur by carrying out this contact at temperatures either above the dew point of the sulphur formed, in which case the sulphur formed is present in the vapour state in the reaction medium coming from the reaction, or at temperatures below the dew point of the sulphur formed, in which case the sulphur is deposited on the catalyst, thereby requiring the sulphur-laden catalyst to be periodically regenerated by vaporizing the sulphur by flushing with a non-oxidizing gas having a temperature of between 200xc2x0 C. and 500xc2x0 0C.
In particular, the oxidation of H2S to sulphur at temperatures above the dew point of sulphur, that is to say, at temperatures greater than approximately 180xc2x0 C., may be carried out in contact with a catalyst consisting of titanium oxide (EP-A-0,078,690), titanium oxide containing an alkaline-earth metal sulphate (WO-A-8302068), titanium oxide containing nickel oxide and optionally aluminium oxide (EP-A-0,140,045), an oxide of the titanium oxide or zirconium oxide or silica type, combined with one or more compounds of transition metals chosen from Fe, Cu, Zn, Cd, Cr, Mo, W, Co and Ni, preferably Fe, and optionally with one or more compounds of precious metals chosen from Pd, Pt, Ir and Rh, preferably Pd (FR-A-2,511,663) or else a thermally stabilized alumina combined with one or more compounds of transition metals, such as those mentioned above, especially Fe, and, optionally, with one or more compounds of precious metals chosen from Pd, Pt, Ir and Rh (FR-A-2,540,092).
The catalysts as mentioned above, consisting of a catalytic phase based on at least one oxide, salt or sulphide of a transition metal and combined with a support consisting of at least one material chosen from alumina, titanium oxide, zirconium oxide, silica, zeolites, silica/alumina mixtures, silica/titanium oxide mixtures and active carbon, which are used for the catalytic oxidation of H2S to sulphur, still have certain inadequacies in prolonged use. In particular, the catalysts which have an alumina-based support are susceptible to changing over time by sulphurization. With regard to the catalysts whose support consists of active carbon, precautions must be taken during their use to prevent oxidation of the support, which is accompanied by a loss of the support. In addition, for these various catalysts, the catalytic phase impregnating the support has a tendency to migrate into the interstices of the support and, thereby, makes it difficult, or indeed often impossible, to recover the metal from the catalytic phase in the spent catalyst. Finally, the abovementioned catalysts have a mediocre thermal conductivity, which means that the temperature within the catalytic beds containing them cannot be effectively controlled by heat exchange with a coolant.
To remedy the drawbacks of the catalysts of the above mentioned type, which are used in processes for the direct catalytic oxidation of H2S to sulphur, carried out in the vapour phase, that is to say at temperatures above the dew point of the sulphur formed and, thus, to obtain a process resulting in an improved sulphur selectivity that continues lastingly over time, the Applicant in FR-A-2,727,101 and WO-A-97/19019 has proposed to carry out the oxidation in contact with a catalyst formed from a silicon carbide support combined with a catalytically active phase containing at least one transition metal, especially Ni, Co, Fe, Cu, Ag, Mn, Mo, Cr, Ti, W and V, in the oxide or salt form and/or in the elemental state.
A process for catalytically oxidizing H2S contained in a gas directly to sulphur containing the following steps:
combining the H2S-containing gas with a gas containing free oxygen in an amount to produce an oxygen-enriched H2S-containing gas having O2/H2S molar ratio ranging from about 0.05 to about 15; and
contacting the oxygen-enriched H2S-containing gas with a catalyst for selective oxidation of H2S to sulphur, wherein the catalyst includes a catalytically active phase combined with a silicon carbide-based support and wherein the active phase of the catalyst consists of at least one oxysulphide of at least one metal selected from the group consisting of iron, copper, nickel, cobalt, chromium, molybdenum and tungsten, at a temperature above the dew point of sulphur formed during H2S oxidation.
The Applicants have found that the desulphurizing activity and sulphur selectivity of the catalyst with a silicon carbide support in the process for direct oxidation of H2S to sulphur, carried out at temperatures above the dew point of the sulphur formed, can further be improved by carrying out the oxidation using an active phase of the catalyst, which consists of one or more oxysulphides of selected transition metals.
The subject of the invention includes, therefore, a process for oxidizing H2S contained at low concentration in a gas directly to sulphur by catalytic means, which process is of the type in which the H2S-containing gas with a gas containing free oxygen, in a suitable amount to give an O2/H2S molar ratio ranging from about 0.05 to about 15, is brought into contact with a catalyst for the selective oxidation of H2S to sulphur, which includes a catalytically active phase combined with a silicon carbide-based support and formed from at least one compound of at least one transition metal, the process being carried out at temperatures above the dew point of the sulphur formed during the H2S oxidation, and it is characterized in that the active phase of the catalyst consists of at least one oxysulphide of at least one metal chosen from iron, copper, nickel, cobalt, chromium, molybdenum and tungsten.
Advantageously, the metal constituting, in the oxysulphide form, the active phase of the oxidation catalyst is iron or a mixture of iron and of at least one of the metals copper, nickel, cobalt, chromium, molybdenum and tungsten. The mixture preferably is mostly iron.
As indicated above, the H2S-to-sulphur oxidation reaction is carried out at temperatures above the dew point of the sulphur produced and, more particularly, at temperatures above about 180xc2x0 C. and possibly going up to about 500xc2x0 C. Preferably, the H2S-to-sulphur oxidation reaction is carried out at temperatures of between about 190xc2x0 C. and about 300xc2x0 C. and more especially between about 200xc2x0 C. and about 260xc2x0 C.
The active phase of the oxidation catalyst, measured by weight of metal, usually represents from about 0.1 to about 20%, more particularly from 0.2% to 15% and more especially from about 0.2% to about 7% of the weight of the catalyst.
The silicon carbide support advantageously forms at least about 40% and, more particularly, at least about 50% of the weight of the oxidation catalyst.
The specific surface area of the H2S-to-sulphur oxidation catalyst may vary quite widely depending on the conditions under which the oxidation process is carried out. Advantageously, the specific surface area, determined by the BET nitrogen absorption method at the temperature of liquid nitrogen (NF X 11-621 standard), may range from about 0.05 m2/g to about 600 m2/g and more particularly from about 0.1 m2/g to about 400 m2/g.
The oxidation catalyst with a silicon carbide support and oxysulphide active phase may be prepared, for example, by carrying out the following. In a first step, an oxidized precursor of the catalyst is prepared by first impregnating a silicon carbide support, which is in the form of a powder, pellets, granules, extrudates or other forms of agglomerates, by means of a solution or of a sol, in a solvent such as water, of the metal or metals desired for the active phase, in the form of salts, and then by drying the support thus impregnated and by calcining the dried product at temperatures that may range from about 250xc2x0 C. to about 500xc2x0 C., which calcining may or may not be carried out in an inert atmosphere.
In a second step, a sulphurized precursor of the catalyst is formed by subjecting the oxidized precursor from the first step to a sulphurization treatment by bringing it into contact with a sulphurizing agent consisting of elemental sulphur and/or of a gas mixture consisting of H2S and of an inert gas, which treatment is carried out at temperatures of between about 250xc2x0 C. and about 500xc2x0 C. and for a time long enough to achieve maximum sulphurization of the metal or metals of the active phase of the catalyst.
In a third step, the catalyst is produced by bringing the sulphurized precursor from the second step into contact, at temperatures below about 500xc2x0 C. and especially between about 250xc2x0 C. and about 450xc2x0 C. and for a time of between about 0.2 and about 3 hours and more especially between about 0.5 and about 2 hours, with an oxygen-containing gas stream consisting of an inert carrier gas containing, in terms of the volume of the inert gas, from about 0.1% to about 40% oxygen and from about 0% to about 50% water, to form the metal oxysulphide phase.
The amount of elemental sulphur used for sulphurizing the calcined impregnated support constituting the oxidized precursor of the catalyst advantageously represents a slight excess, for example, an excess ranging up to about 300 mol % with respect to the stoichiometric amount corresponding to maximum sulphurization of the metal or metals of the active phase of the catalyst. The duration of the sulphurization treatment with elemental sulphur is advantageously between about 0.5 and about 4 hours.
The gas mixture, consisting of H2S and inert gas, used as sulphurizing agent advantageously contains between 0.2% and 30% H2S by volume. The duration of the sulphurization treatment with the gas mixture containing H2S is generally between about 0.5 and about 15 hours.
According to one embodiment, and in order to form the metal oxysulphide phase, the oxygen-containing gas stream brought into contact with the sulphurized precursor of the catalyst is air. According to another embodiment, the oxygen-containing gas stream consists of an inert gas, for example, nitrogen, argon, helium or mixtures of at least two of these gases, containing, by volume of inert gas, about 0.1% to about 3%, more especially about 0.3% to about 1.5%, oxygen and about 0% to about 50%, more especially about 0.5% to about 30%, water.
The silicon carbide used for forming the support for the H2S-to-sulphur oxidation catalyst may consist of any one of the known silicon carbides, as long as it has the required specific surface area characteristics, namely a specific surface area, determined by the BET nitrogen adsorption method, ranging from about 0.05 m2/g to about 600 m2/g.
As examples, the silicon carbide may be prepared using any one of the techniques described in EP-A-0,313,480 (corresponding to U.S. Pat. No. 4,914,070), EP-A-0,440,569, EP-A-0,511,919, EP-A-0,543,751, EP-A-0,543,752 and FR-A-2,758,279.
The gas containing free oxygen used for the oxidation to sulphur of the H2S contained in the gas to be treated is generally air, although it is possible to use pure oxygen, oxygen-enriched air or other mixtures which consist, in various proportions, of oxygen and of at least one inert gas and which are other than air and oxygen-enriched air.
The gas containing free oxygen and the gas to be treated containing H2S may be brought separately into contact with the oxidation catalyst. However, to obtain a very homogeneous gaseous reaction mixture during contact with the catalyst, it is preferable first to mix the gas to be treated containing H2S with the gas containing free oxygen, and to bring the mixture thus produced into contact with the oxidation catalyst.
As indicated above, the gas containing free oxygen is used in an amount suitable for providing an O2/H2S molar ratio ranging from about 0.05 to about 15, more particularly from about 0.1 to about 10, in the reaction mixture coming into contact with the H2S-to-sulphur oxidation catalyst. Advantageously, the molar ratio is between about 2.5 and about 8, thereby stabilizing the oxysulphide active phase of the catalyst.
Most particularly, the H2S-to-sulphur oxidation reaction is carried out at temperatures of between about 200xc2x0 C. and about 260xc2x0 C. with an O2/H2S molar ratio ranging from about 3 to about 7.
The times during which the gaseous reaction mixture is in contact with the oxidation catalyst may range from about 0.5 to about 20 seconds and preferably from about 1 to about 10 seconds, these values being given under standard pressure and temperature conditions (STP contact time).
The gas having a low H2S content, which is treated by the process according to the invention, may come from various sources. In particular, such a gas may be a natural gas having a low H2S content or an acid gas resulting from the desulphurization of a natural gas or, alternatively, a gas coming from the gasification of coal or of heavy oils, or even a gas resulting from the hydrogenation of a waste gas, for example, a waste gas from a sulphur plant, containing sulphur compounds such as SO2, mercaptans, COS and CS2, which can be converted into H2S by the action of hydrogen or water vapour.
The gas to be treated may also be a gas resulting from the treatment, in contact with a Claus catalyst capable of promoting the sulphur-formation reaction between H2S and SO2, of a gaseous effluent containing H2S and SO2 in an H2S/SO2 molar ratio of greater than 2/1, and such that the resulting gas contains above all H2S and no or very little SO2 as sulphur compounds. The process according to the invention can be applied to the treatment of gas having an H2S content of less than about 25% by volume and especially between about 0.001% and about 20% by volume. In the most common cases, the content is between about 0.005% and about 10% by volume and more particularly ranges from about 0.01% to about 5% by volume. The gas to be treated may also contain organic sulphur compounds, such as mercaptans, COS and CS2, in overall concentrations that may range up to approximately 1% by volume. It is possible, using the process according to the invention, to treat gases containing H2S in a concentration greater than about 25% by volume. However, in such a case, it is preferred to use the conventional sulphur-production processes comprising a thermal reaction step.
The H2S-containing gas, which is oxidized in contact with the catalyst with a silicon carbide support, may be free of water or substantially free of water or, on the contrary, it may contain quite a high amount of water. Thus, it is possible according to the invention to treat an H2S-containing gas which has a water content that can range from about 0% to approximately about 50%.
In the catalytic H2S-to-sulphur oxidation process according to the invention, which is carried out in the vapour phase, that is to say at temperatures above the dew point of the sulphur formed during the H2S oxidation reaction, the sulphur is present in the vapour state in the reaction medium, which is in contact with the catalyst and which is collected at the outlet of the catalytic oxidation zone.
The process according to the invention may be carried out in a single oxidation zone containing the oxidation catalyst, especially when the H2S content of the gas to be treated is not greater than about 5% by volume, or else in a plurality of oxidation zones placed in series, each containing the oxidation catalyst, especially when the H2S content of the gas to be treated is greater than about 5% by volume. The single oxidation zone or each of the zones of the plurality of oxidation zones operate at temperatures lying within the intervals defined above. Each of the oxidation zones works in the temperature range corresponding to substantially optimal selectivity of the catalyst for the formation of sulphur.
Since the H2S-to-sulphur oxidation is highly exothermic, it may be necessary to cool the catalyst contained in the single oxidation zone or in each of the zones of the plurality of oxidation zones, to maintain the temperature in the zone at the desired value or within the desired range corresponding to the optimum sulphur selectivity of the catalyst. It is possible, in particular, to cool the catalyst using a cold fluid, for example, water or another fluid flowing in indirect heat exchange with the catalyst, within the latter. The cooling may also be carried out by placing the catalyst in a tube reactor, consisting of one or more tubes arranged in a shell, with, for example, the catalyst being in the tube or the tubes and a cold fluid, for example, water or another fluid flowing in the shell outside the tube or tubes or vice versa.
An effluent gas laden with sulphur vapour is collected at the outlet of the single oxidation zone or of each of the zones of the plurality of oxidation zones in series, which gaseous effluent, before any subsequent H2S removal, is passed through a sulphur separation zone in which it is stripped of most of the sulphur that it contains by condensation. When the H2S-containing gas is treated by passing through a plurality of oxidation zones in series, which contain the oxidation catalyst with a silicon carbide support, only a fraction of the H2S contained in the gas to be treated is oxidized to sulphur in each of the zones, by injecting into the zone in question, preferably as a mixture with the gas brought to this zone, the appropriate amount of the gas containing free oxygen to carry out this sulphur oxidation. The amount of H2S subjected to sulphur oxidation in each of the zones, which represents a fraction of the total amount of H2S in the gas to be treated, is advantageously between about 2% and about 5% by volume of the gas to be treated and the number of catalytic oxidation zones is chosen so that the gas to be treated arriving at the final catalytic zone contains at most about 5% H2S by volume.
If required, the gaseous effluent collected at the outlet of the single oxidation zone or at the outlet of the final zone of the plurality of oxidation zones in series may be subjected to an additional purification step after separating out the sulphur that it may contain, the treatment depending on the nature of the gaseous sulphur compounds remaining in the effluent.
Advantageously, the H2S oxidation catalyst used according to the invention may be formed directly, in each oxidation zone employed for the treatment of the H2S-containing gas, as an operation prior to the H2S oxidation operation. To do this, an oxidized precursor of the catalyst, prepared as indicated above, is placed in each oxidation zone and, by acting directly in the oxidation zone, the oxidized precursor is subjected to a sulphurization treatment to produce a sulphurized precursor and then the sulphurized precursor undergoes an oxidizing treatment using an oxygen-containing gas stream to form the catalyst with an oxysulphide phase, the sulphurization treatment and the oxidizing treatment corresponding to the second and third catalyst preparation steps described above, respectively. The sulphurization treatment may especially be carried out using the H2S-containing gas to be treated as a sulphurizing agent.
According to the invention, when the activity of the oxidation catalyst, during the H2S oxidation phase, falls below a predetermined threshold, the efficiency of the catalyst, that is to say its desulphurizing activity and its sulphur selectivity, may be restored by subjecting the deactivated catalyst to an oxidizing treatment under the conditions of the third catalyst preparation step that were described above.